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SubjectRe: [RFC PATCH 18/18] Documentation: add document for pte_ref
Hi Qi,

On Fri, Apr 29, 2022 at 09:35:52PM +0800, Qi Zheng wrote:
> +Now in order to pursue high performance, applications mostly use some
> +high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
> +These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
> +physical memory for the following reasons::
> +
> + First of all, we should hold as few write locks of mmap_lock as possible,
> + since the mmap_lock semaphore has long been a contention point in the
> + memory management subsystem. The mmap()/munmap() hold the write lock, and
> + the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
> + madvise() instead of munmap() to released physical memory can reduce the
> + competition of the mmap_lock.
> +
> + Secondly, after using madvise() to release physical memory, there is no
> + need to build vma and allocate page tables again when accessing the same
> + virtual address again, which can also save some time.
> +

I think we can use enumerated list, like below:

-- >8 --

diff --git a/Documentation/vm/pte_ref.rst b/Documentation/vm/pte_ref.rst
index 0ac1e5a408d7c6..67b18e74fcb367 100644
--- a/Documentation/vm/pte_ref.rst
+++ b/Documentation/vm/pte_ref.rst
@@ -10,18 +10,18 @@ Preface
Now in order to pursue high performance, applications mostly use some
high-performance user-mode memory allocators, such as jemalloc or tcmalloc.
These memory allocators use madvise(MADV_DONTNEED or MADV_FREE) to release
-physical memory for the following reasons::
-
- First of all, we should hold as few write locks of mmap_lock as possible,
- since the mmap_lock semaphore has long been a contention point in the
- memory management subsystem. The mmap()/munmap() hold the write lock, and
- the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
- madvise() instead of munmap() to released physical memory can reduce the
- competition of the mmap_lock.
-
- Secondly, after using madvise() to release physical memory, there is no
- need to build vma and allocate page tables again when accessing the same
- virtual address again, which can also save some time.
+physical memory for the following reasons:
+
+1. We should hold as few write locks of mmap_lock as possible,
+ since the mmap_lock semaphore has long been a contention point in the
+ memory management subsystem. The mmap()/munmap() hold the write lock, and
+ the madvise(MADV_DONTNEED or MADV_FREE) hold the read lock, so using
+ madvise() instead of munmap() to released physical memory can reduce the
+ competition of the mmap_lock.
+
+2. After using madvise() to release physical memory, there is no
+ need to build vma and allocate page tables again when accessing the same
+ virtual address again, which can also save some time.

The following is the largest user PTE page table memory that can be
allocated by a single user process in a 32-bit and a 64-bit system.
> +The following is the largest user PTE page table memory that can be
> +allocated by a single user process in a 32-bit and a 64-bit system.
> +

We can say "assuming 4K page size" here,

> ++---------------------------+--------+---------+
> +| | 32-bit | 64-bit |
> ++===========================+========+=========+
> +| user PTE page table pages | 3 MiB | 512 GiB |
> ++---------------------------+--------+---------+
> +| user PMD page table pages | 3 KiB | 1 GiB |
> ++---------------------------+--------+---------+
> +
> +(for 32-bit, take 3G user address space, 4K page size as an example;
> + for 64-bit, take 48-bit address width, 4K page size as an example.)
> +

... instead of here.

> +There is also a lock-less scenario(such as fast GUP). Fortunately, we don't need
> +to do any additional operations to ensure that the system is in order. Take fast
> +GUP as an example::
> +
> + thread A thread B
> + fast GUP madvise(MADV_DONTNEED)
> + ======== ======================
> +
> + get_user_pages_fast_only()
> + --> local_irq_save();
> + call_rcu(pte_free_rcu)
> + gup_pgd_range();
> + local_irq_restore();
> + /* do pte_free_rcu() */
> +

I see whitespace warning circa do pte_free_rcu() line above when
applying this series.

Thanks.

--
An old man doll... just what I always wanted! - Clara

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